The invention relates generally to the field of embossing. More particularly, the invention concerns an apparatus and method of embossing thin, low density polyethylene (LDPE) film using a temperature controlled die and an opposing unheated die.
Currently, radio frequency (RF) heating is the preferred industry technique for embossing thin film materials. In a typical RF process, a single die is connected to a high voltage RF source. The film material or web to be embossed is then arranged on a grounded metallic plate and positioned for engaging contact by the single die. RF generates heat in the web, directly or indirectly, which then softens the material to near its melting point. With the associated pressure from the dies on the film material or web, a permanent impression of the intaglio on the dies is transferred to the web. A well-known shortcoming of the current RF technique is that it is extremely sensitive to the embossing material. In particular, film materials like polyethylene, have proven to be very poor for absorbing RF energy. Moreover, the embossing process cycle time using RF is exceedingly long because of ineffectiveness of certain materials in absorbing RF energy. Further, another shortcoming of present RF embossing techniques is that the quality of the embossed image is not predictably consistent over a range of film material properties. This is particularly true with respect to embossing thin, low density polyethylene and similar film materials, such as polyester and polystyrene.
U.S. Pat. No. 3,584,572 discloses a method and apparatus for simultaneously heat stamping, embossing and cutting a workpiece, such as plastic sheet material, that uses a single die. However, a shortcoming of this development is that it uses a single die for cutting and embossing the film and the embossed image is formed only along an edge of the workpiece.
Moreover, heretofore, embossed thermoplastic films such as polyethylene, polypropylene, polybutene-1, polyvinyl chloride, and other flexible thermoplastics normally extruded into film have been made by various methods. One method used to prepare embossed thermoplastic film is to extrude the thermoplastic material, e.g., polyethylene, from a conventional slot extrusion die onto a continuously moving, smooth, cooled casting surface, e.g., a chill roll. The embossed films made by the above described process often produce poor roll conformation, i.e., have hard or soft spots, wrinkle or sag due to the inability to distribute zones of thickness variation across the width of the roll of embossed film. Embossed film rolls of poor conformation produces problems when running the film through fabricating machines or through a film printing apparatus.
An example of a method and apparatus for producing film according to the foregoing slot die-chill cast roll technique is shown in U.S. Pat. No. 3,374,303.
Another technique used for embossing plastic film heretofore has been the utilization of a heated engraved embossing roll used in conjunction with a backup roll. The resultant embossed film usually has a very shallow and poorly defined pattern. An example of an apparatus and process for carrying out embossing of this type is shown in U.S. Pat. No. 3,176,058.
One of the drawbacks when using the heated drum or roll method to apply heat to the film prior to embossing is the difficulty in heating the film sufficiently so that it will take a deep, permanent embossed pattern which is a true reproduction of the engraved surface of the engraving roll.
Thus, it can be seen that the film embossing art is in need of the apparatus and method of the present invention whereby thermally deformable plastic material, such as low density polyethylene can be controllably heated to near its melting point and embossed, and then quickly cooled to obtain a density depression in the plastic web material forming a deep, permanent embossed pattern on the material. Moreover, the invention enables wide variation in control of the parameters which will permit variation in the physical properties of the embossed material, i.e., modulus, surface gloss, embossed pattern depth, tensile strength, and impact strength.
It is, therefore, an object of the invention to provide a method of embossing that is more effective than present techniques.
Another object of the invention is to provide a method of embossing that utilizes a pair of dies using a controlled heat source to control the heat generated in the plastic material.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, an apparatus for embossing indicia on a plastic web material comprises:
a rigid upright frame;
a source of power associated with said frame;
a first, movable embossing element mounted for movement on said frame, said first embossing element comprising a heat transferable member heated by said source of power, a mounting plate member having a first side and a second side, said first side being affixed to said heat transferable member, and a plurality of male dies affixed to said second side of said mounting plate member, each one of said plurality of male dies having an intaglio design thereon; and
a second, stationary embossing element supportably alignable in said frame with said first embossing element, said second embossing element comprising a plurality of female dies each one of which to receive a corresponding one of said plurality of male dies, one of said plurality of female dies having a corresponding intaglio design thereon and a mounting plate for supporting said plurality of female dies.
In another aspect of the invention, a method of embossing thermally deformable plastic web material comprising the steps of:
(a) providing an embossing unit comprising a first, movable embossing element having a heat transferable member, a mounting plate member supported affixed to said heat transferable member and a plurality of first dies having a predetermined male pattern affixed to said mounting plate member; and a cooperating, second embossing element alignable with said first embossing element, said second embossing element having a plurality of second dies having a predetermined female pattern for receiving said predetermined male pattern of said first dies, and, a support plate for supporting said plurality of second dies;
(b) providing power to said heat transferable member, said heat transferable member heated to a predetermined temperature;
(c) arranging said thermally deformable plastic web material on said plurality of dies;
(d) moving the first embossing element towards said second embossing element so that said plurality of first dies engageably contact said thermally deformable plastic web material;
(e) maintaining said plurality of first dies in contact with said thermally deformable plastic web material for a predetermined period of time thereby forming an embossed image on said thermally deformable plastic web material corresponding to said predetermined male and females patterns on said plurality of first dies and said plurality of second dies; and,
(f) moving the first, movable embossing element away from the thermally deformable plastic web material so as to permit cooling of the thermally deformable plastic web material.
The present invention has the following advantages: the embossed image has a predictably high quality; the process enables multiple logos to be produced on a single web; the processing time is much shorter; the process is effective for a range of materials, particularly polyethylene; and it is easier to operate in a production.